I've never understood the uncapped header thing every car I have ever driven without mufflers has been completely gutless with no bottom end power at all. The cars were driven to the exhaust shop with just headers installed then when the mufflers and tail pipes were installed you could feel the power increase so why would you want to uncap the exhaust? I think it has to do with the back pressure on the valves there is a condition known as over exhausting this is when the exhaust pipes are too large for the engine, turbo charged engines are not as sensitive to this as naturaly aspirated engines because the turbo provides some back pressure. I once drove a turbo charged car with no mufflers to the exhaust shop and on the way back you could feel the power increase with the full exhaust system fitted. Even my 2 stoke motocross bikes are hopeless with the muffler removed so I feel any engine will produce maximum power with the correct exhaust system connected I have even heard the exhaust valves can be damaged without the correct amount of back pressure.